Method and apparatus for liquid/solid separation such as dewatering particulate solids and agitation leaching

ABSTRACT

Methods and apparatus for liquid/solid separation for use in applications such as dewatering fine particulate solids, and recovery of valuable metals from ore in a leaching process are provided. One application relates to methods of agitation leaching of metals such as gold from gold-bearing feedstock. A slurry is formed in a tank by agitation, and allowed to settle. A filter bed forms to drain the liquid from the tank, and a vertical screen pipe such as a well point addresses the formation of an impervious film on the upper surface of the filter bed.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefits, under 35 U.S.C. § 119(e),of U.S. Provisional Application Ser. No. 61/883,411 filed Sep. 27, 2013entitled “Method and Apparatus for Agitation Leaching” which isincorporated herein by this reference.

TECHNICAL FIELD

The invention relates to methods and apparatus for liquid/solidseparation for use in applications such as dewatering particulatesolids, and recovery of valuable metals from ore in a leaching process.In one aspect the invention relates to methods of agitation leaching ofmetals such as gold from gold-bearing feedstock.

BACKGROUND

Separating particulate solids from a liquid has a multiplicity ofapplications. They include dewatering of particulate slurries andseparation of liquid from solids in the leaching process. The leachingof constituents from particulate mineral materials is practisedutilizing a wide variety of materials and equipment. Leaching proceduresare particularly useful for the recovery of metals from particulatemineral ores, such as gold, silver, copper and uranium ores. Thedominant process for the extraction of such metals from ores is leachingwith lixiviants. Typical leaching methods have a number of drawbacks, inparticular the need to either grind the ore finely for continuousagitation leaching or utilize batch leaching methods on coarsermaterial.

Tank leaching is typically a continuous process, while vat leaching isoperated in a batch fashion. Tank leaching is commonly used to extractgold and silver from ore. Tank leaching differs from vat leaching inthat in tank leaching the material is ground sufficiently finely to forma slurry which can flow under gravity or when pumped, whereas in vatleaching typically a coarser material is placed in the vat for leaching.The tanks in the tank leaching method are typically equipped withagitators to keep the solids in suspension in the tanks and improve thesolid to liquid to gas contact. Baffles can be provided to increase theefficiency of agitation and prevent centrifuging of slurries in circulartanks. The vats in vat leaching usually do not contain such equipment.In a tank leach the slurry is agitated, while in a vat leach the solidsremain stationary in the vat, and solution is moved, so typically theretention time required for vat leaching is more than that for tankleaching to achieve the same percentage of recovery of the valuablematerial being leached.

Tank and vat leaching both involve placing the ore, after size reductionand classification, into the tanks or vats at ambient operatingconditions containing a leaching solution and allowing the valuablematerial to leach from the ore into solution. In tank leaching theground, classified solids are already mixed with water to form theslurry, and this is pumped into the tanks. Lixiviants are added to thetanks to achieve the leaching reaction. In a continuous system theslurry will then either overflow from one tank to the next, or be pumpedto the next tank. Ultimately the pregnant solution is separated from thesolids using some form of liquid/solid separation process, and thesolution passes on to the next phase of recovery. In vat leaching thesolids are loaded into the vat and, once the vat is full it is floodedwith a leaching solution. The solution drains from the tank, and iseither recycled back into the vat or is pumped to the next step of therecovery process.

The factors which affect extraction efficiency are: i) Retentiontime—the time spent in the leaching system by the solids. This iscalculated as the total volumetric capacity of the leach tank(s) dividedby the volumetric throughput of the solid/liquid slurry. ii) Particlesize—The ore is ground to a size that exposes the desired mineral to theleaching agent. In tank leaching this must be a size that can be fullymixed and suspended by the agitator. In vat leaching this is the sizethat is the most economically viable, balancing recovery against theincreased cost of processing the material. iii) Slurry density—Theslurry density (percent solids) determines retention time. The settlingrate and viscosity of the slurry are functions of the slurry density.The viscosity, in turn, controls the gas mass transfer and the leachingrate. iv) Dissolved gas—Gas, typically oxygen, may be injected into thesolution to obtain the desired dissolved gas levels. vi) Reagents areadded and the appropriate amount of reagents maintained throughout theleach circuit to maximize the metal recovery. v) Temperature—impacts thereaction kinetics. vi) Leach-inhibiting elements such aslixiviant-consuming minerals or carbonaceous materials.

The conventional knowledge says that the maximum particle size foragitation leaching should be much less than 1 mm in diameter to permitmaximum recovery in a reasonable retention time, as well as allowingfully homogeneous mixing. In gold leaching by carbon capture, usingfinely ground particles allows carbon separation. Such a fine particlesize requires expensive grinding.

The foregoing examples of the related art and limitations relatedthereto are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those of skill inthe art upon a reading of the specification and a study of the drawings.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods which aremeant to be exemplary and illustrative, not limiting in scope. Invarious embodiments, one or more of the above-described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

The present invention therefore provides a method and apparatus forliquid/solid separation. According to one aspect the invention providesa method and apparatus for dewatering particulate solids.

There is therefore provided an apparatus for separating a liquid fromparticulate solids, comprising: i) a tank for containing the particulatesolids and liquid as a slurry; ii) an input to the interior of the tankfor introduction of the particulate solids and liquid into the tank;iii) a liquid outlet passage communicating with the interior of thetank; iv) an agitator suspended within the tank for forming a suspensionof the particles in said liquid; v) the tank having a lower section forformation of a filter bed to drain liquid from the tank; and vi) ascreen pipe extending upwardly through the filter bed area andcommunicating with the output passage to receive at its upper end a flowof liquid from above the lower section which is carried downwardly toflow out through the filter bed section to the outlet passage ordirectly to the outlet passage. The screen pipe may be a well point orwell screen.

According to a further aspect the present invention further provides amethod of separating a liquid from particulate solids, comprising thesteps of: i) providing an apparatus as described above for dewateringparticulate solids, comprising: ii) introducing the particulate solidsand liquid into the tank; iii) agitating the particulate solids andliquid to form a slurry; iv) ceasing the agitation to allow the slurryto settle, thereby forming a graded filter bed to drain liquid from thetank; v) using the screen pipe to transfer liquid from above the filterbed to within the filter bed or directly to the outlet passage; vi)draining the liquid from the particulate solids; and vii) removing theparticulate solids. An amount of coarser particulate solids may be addedto the particulate solids and liquid in the tank, wherein the coarserparticulate solids are suitable for forming a filter bed. The agitatormay be a variable speed agitator and the agitation is slowed prior tothe settling step.

The apparatus may be used for agitation leaching of mineral-bearing ormetal-bearing particles, a significant portion of which may be 1 mm indiameter or greater, wherein the particulate material comprises gold,silver, copper or uranium and the liquid comprises concentrations oflixiviant.

The method may be used for agitation leaching of mineral-bearing ormetal-bearing particles, a significant portion of which may be 1 mm indiameter or greater, wherein the particulate material comprises gold,silver, copper or uranium and the liquid comprises concentrations oflixiviant.

In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to thedrawings and by study of the following detailed descriptions.

BRIEF DESCRIPTION OF DRAWINGS

Exemplary embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be considered illustrative rather than restrictive.

FIG. 1 is a left side perspective view of an installation for carryingout the process of the invention.

FIG. 2 is a right rear perspective view of the installation shown inFIG. 1.

FIG. 3 is an elevation view of the installation shown in FIG. 1.

FIG. 4 is a schematic diagram taken in cross-section along lines 4-4 ofFIG. 1.

FIG. 5 is an elevation view in cross-section along lines 4-4 of FIG. 1of the leaching tank.

FIG. 6 is an elevation view of the settling and storage tank.

DESCRIPTION

Throughout the following description specific details are set forth inorder to provide a more thorough understanding to persons skilled in theart. However, well known elements may not have been shown or describedin detail to avoid unnecessarily obscuring the disclosure. Accordingly,the description and drawings are to be regarded in an illustrative,rather than a restrictive, sense.

Improvements in methods and apparatus for liquid/solid separation aredescribed below which have applications in dewatering particulatesolids. Particularly useful application is found in recovery of valuablemetals from ore in a leaching process, and more particularly methods ofagitation leaching of metals such as gold from gold-bearing feedstock.The apparatus utilizes slotted, apertured, fenestrated, perforated ormesh vertical tubes or pipes. One suitable form of such tube or pipe isreferred to as a well point. These are vertical tubes or pipes,typically stainless steel or PVC, with a continuous array of openings,which may be a screen and/or gauze, which allow the flow of liquid intoand along the central passage of the tube but prevent the entry ofparticles having greater than a selected diameter. They also share suchfeatures with well screens. All of these devices will be referred tointerchangeably herein as well points, well screens or more generallyscreen pipes.

Continuous Agitation Leaching

The applicant has developed a new process and apparatus which in oneapplication, to agitation cyanide leaching of gold-bearing particles,may includes the following features:

leaching of particles up to approximately 2 mm in size or greater;

high concentrations of cyanide, up to 10 times typical concentrations;

a variable speed agitation;

formation of a filter bed to drain the cyanide/gold solution from thetank; and

use of a vertical screen pipe such as a well point to deal with theformation of an impervious film on the upper surface of the filter bed.

The conventional knowledge says that the maximum particle size foragitation leaching should be much less than 1 mm. The applicant hasdeveloped a new process that can process larger particles—2 mm orgreater—and process them more quickly. The leaching process is intensiveas the concentration of cyanide is on the order of 10 times the usualconcentration. A variable speed motor 41 for the agitator 40 may beused, operating at the lowest speed possible which puts the coarsestparticles into motion at the bottom of the tank, resulting insignificantly less component wear than high speed homogeneous mixing.The agitator then slows so that heavier particles sink to the bottom ofthe tank and settle, eventually forming a graded filter bed with coarseparticles at the bottom and finer particles at the top. The agitationblades remain above the bed in the liquid slurry. This graded bed servesas the filter to drain the cyanide/gold solution from the tank.

With reference to FIGS. 1, 2 and 6, a feed settling and storage tank 10is fed gold-bearing crushed concentrate slurry, for example recoveredfrom a centrifugal concentrator, through feed pipe 17. Solids areallowed to settle and excess water is drained off from the concentrate,such as through a bleed valve 15. Overflow pipe 13 may re-directoverflow from tank 10. A water circulation line may be provided at 12.At selected intervals, when the tank 10 is full of slurry which has beendewatered to the desired degree, the outlet valve 14 is opened and theconcentrate is pumped from tank 10 via pipes 16 in the form of a slurry11 (FIG. 4) into substantially empty leaching tank 20.

Cylindrical leaching tank 20, shown in FIG. 5 has lid 21, vertical sides25 and bottom 23. Feed pipe 62 carries the feed concentrate slurry frompipe 16 onto channel 63 and then into the interior of tank 20. Feed pipe62 also carries the cyanide solution into tank 20. Pipe 64 is one ofseveral adjacent pipes through which reagents can be added to the tank20. A reagent addition pipe (not shown) and a feed pipe 18 forrecirculation also enters tank 20 onto channel 63, adjacent to pipe 62.Agitator 40 driven by variable speed motor 41 is suspended from frame 43and has blades 42 rotated on shaft 45. Level sensor 70 senses the levelof solids or cyanide solution within tank 20. A hydrogen cyanide sensor72 senses the presence of hydrogen cyanide in tank 20 as a safetymeasure since the gas, produced as part of the leaching process, can beexplosive and highly toxic for humans when present in excessivequantities. Baffles 28 are secured to and extend from wall 25 to reducerotation of the slurry.

Well points 60 are threaded into the bottom 23 of tank 20 andcommunicate by valved connection with piping 52 which removes thepregnant solution. Tailings discharge port 30 communicates with atailings discharge pipe and is opened or closed by valve actuator 74, oralternately a manual override 76. Pots or static screens 48 are sealedto the bottom 23 of tank 20 and communicate through screens 49 with theinterior of tank 20 and through pipe 52 to discharge the pregnantsolution. Oxygen inlet 66 injects oxygen into tank 20.

Leaching tank 20 is filled with concentrate until the level of solids inthe slurry is somewhat below the blades 42 of agitator 40. A cyanidesolution, such as sodium cyanide, with a high concentration of cyanide,up to 10 times typical leaching concentrations, is added to the tank 20via pipe 16 and discharge pipe 62, up to level 22, leaving the solid toliquid ratio at about 30-40%. Preferably finely divided oxygen bubblesare added directly to the solution via inlet 66 and dissolved leaving aminimum 10 parts per million dissolved oxygen in the solution to improvethe reaction. Lid 21 allows an over-pressure to build up in tank 20 whenoxygen is introduced, to increase the dissolution of oxygen in thesolution.

Agitator 40 is started rotating for a number of hours, typically up to18-20 depending on the ore characteristics. Preferably blades 42 asshown in FIG. 5 are configured to cause a shearing motion through thesolution which aids the leaching. For example blades 42 may betriangular in cross-section angled downwardly with the sharp end of thetriangle forming the attacking edge of the blade. Baffle 28 reducesrotation of the slurry to increase the mixing effect. Approximately 5minutes before the agitator is stopped, coagulant may be added to thesolution and mixed for a short period such as 2 minutes, followed by theaddition of flocculant which is mixed for about 3 minutes. The agitatoris then slowed and stopped over several minutes and the solids areallowed to settle. The heavier particles settle most quickly to form afilter bed 50, followed by the lighter particles. A typical settlingtime is 30 minutes.

As the finest particles settle out of the slurry, a thin film forms at54 over the bed 50. To allow liquid to easily flow through bed 50, wellpoints or screens 60 extend through the fines film that coats the filterbed at 54. A suitable type of well point for example has a roughly 2″inside diameter by about 3′ long with 7/1,000″ to 10/1,000″ slotopenings as manufactured by Johnson Screens. One size used successfullywas a size 2P Sand Point Tube, 60 wire model 304 Stainless Steel screenwith 10/1,000 inch screen openings, model number 936. Well screens mayalso be used. The liquid flows through the well point and into bed 50when the valve connecting the well point 60 to pipe 52 (not shown) isshut, and directly out pipe 52 when open.

The well points 60 are first cleared by back flushing, by reversingvacuum pump 56, and then recirculating liquid through the well points byvacuum pump 56 with the well point valves open, central draw throughscreened pots 48 closed, and recirculation pipe 18 open. Once the wellpoints are clear, after about 3 minutes of recirculation, the well pointvalves are closed and the pregnant solution is drawn through the wellpoints into bed 50 and out bottom screened pots 48 by vacuum pump 56into outlet pipe 52 and then preferably the pregnant solution isrecirculated through pipe 18 back into the top of tank 20 and throughthe well points 60 and filter bed 50 for a further short period of time,such as 5-6 minutes, to clarify the solution, until the clarity of theeffluent is clear enough for further processing. The clarified pregnantliquid can then be pumped directly by vacuum pump 56 through the wellpoints 60 by opening the valved connection to pipe 52, or indirectlythrough bed 50 and pots 48 into outlet pipe 52, and pumped to a holdingtank in preparation for electrowinning or other processing method.

Once the liquid from tank 20 is removed, the concentrate bed 50 isrinsed and the liquid removed to recover any dissolved gold. This can bedone by first adding barren cyanide or water to the tank 20, up to thelevel of the agitator 40, or above that level if agitation is desired,and draining the liquid through the bed 50 to pipe 52. Cyanide is thenremoved from the bed 50. This may be done by carrying out a clean waterrinse which is carried out pipe 52 to a different destination than thepregnant solution or using other known cyanide removal methods. Thesolids are then drained by adding water above the level of the agitator40, agitating for a short period such as 5 minutes, and then allowingthe slurried concentrate to flow by gravity through discharge port 30.

EXAMPLE 1 Cyanide Leaching of Gold

A successful pilot plant as described above was set up to test theinvention for cyanide leaching of gold from gravity concentrated ore.Feed storage tank 10 had a volume of 2.3 m³. Leach tank 20 had a volumeof 8 m³, which volume can be varied depending on throughput. Openings inthe well points 60 and bottom screens 49 were sized at 25 microns.Agitator 40 rotational speed was set at 100 to 120 rpm. Settling timewith the agitator 40 stopped was 30 minutes to settle particles smallerthan 75 microns.

The pilot plant provided successful leaching of the gold with a maximumleaching retention time of 18 to 20 hours, out of a 24-hour operatingcycle. The maximum daily throughput was 4 tons per day of solids, with 3tons per day being typical. Feed particle size was maximum 6 mm, with 2mm being typical. This is typical particle size for the concentrate froma Falcon Concentrator. The concentration of gold in the feed solids wasabout 800 grams/tonne, to 900 grams/tonne. A concentration of cyanide inthe leaching solution of 10,000 ppm to about 25,000 ppm, and typically20,000 ppm was used. The concentration is selected depending on the oreproperties. Leach tank 20 solids concentration was a maximum 40 weightpercent with 30 weight percent being typical. Reagents were used, withNaCN used for setting the concentration of CN (cyanide), pH Controlprovided by lime or NaOH, and dissolved Oxygen (O²⁻) control by spargedoxygen gas or air. Four well points and four screened pots 48 were usedto drain the liquid although a smaller number would also have worked, asfew as two well points and one screened pot 48.

The trial results showed an efficiency of recovery of around 99.3%, forexample with a head grade of 900 grams/ton and tailings of about 6 gramsper ton of gold.

The disclosed method when applied to cyanide leaching produces a clearereffluent through the graded sand filter. This is an advantage for theelectrowinning that occurs after leaching. It provides betterliquid/solid separation. The competing technologies discard the finesbefore leaching commences or have an inefficient and incomplete way ofseparating the liquid from the solids. The present system is easilyscalable, both upwardly and downwardly, so is cost-effective at bothsmall scale and at large scale unlike competing systems. It is readilyacceptable to industry being based on conventional mixing technologythat is widely used and understood in the extraction of gold and useswidely available components.

Dewatering of Particulate Solids

It has been found that generally the same apparatus and methodology usedfor cyanide leaching as described above can be used more generally fordewatering of particulate solids of many kinds where such solids wouldnot otherwise be free draining. While the method can be used with aslurry of just fine particulate materials, it has been foundparticularly effective where a seed amount of coarser, heavierparticulate material is also added to form the filter bed.

The following tests as set out in examples 2 and 3 were performed on alaboratory-size, scaled-down version of the apparatus used in Example 1and shown in FIG. 5. The liquid/solid separation method was alsoessentially the same. Where a coarse filter bed material was added tothe slurry, silica sand particles of mixed coarseness were used as thebed material. Such coarse particles were screened to range from 2 mm to4 mm in diameter and may be different sized fractions depending on thefeed material characteristics.

As in the leaching method described above, the solids to be dewatered,with or without the seed filter bed material according to the particulartest, were added to the tank 20 along with the desired amount of water,with the surface level of the liquid lying above the agitator. Agitator40 was started rotating for several minutes at low speed (40-60 hz or800 to 1200 rpms) until the solid particles were in suspension. Shortlybefore the agitator was stopped, flocculant was added to the mixture.The type of flocculant such as polymer flocculant, is determined by thetype of particulate solids to be agglomerated. The agitator 40 was thenslowed and stopped over four minutes and the solids allowed to settle.The heavier particles settle most quickly to form a filter bed 50,followed by the lighter particles. Hence if coarser heavier particleswere added as a seed bed those settled to form the filter bed first. Aswith the leaching method described above, when the finest particlessettle out of the slurry, a thin film forms at 54 over the bed 50. Toallow liquid to continue to flow through bed 50, well points or screens60 extend through the fines film that coats the filter bed at 54.

The well points are then cleared by back flushing, by reversing vacuumpump 56, and then recirculating liquid down through the well points byvacuum pump 56 with the well point valves open and central draw throughscreened pots 48 closed. Once the well points are clear, after about 3minutes of recirculation, the well point valves are closed and theliquid is drawn through the well points into bed 50 and out bottomscreened pots 48 by vacuum pump 56. Initially the filtered liquid isthen recirculated by sending it back above the filter bed into the topof tank 20 and then through the well points 60 and filter bed 50 for afurther short period of time, such as 5-6 minutes, to clarify theliquid, until the clarity of the effluent is clear enough for furtherprocessing. The liquid can then be sucked directly by vacuum pump 56through the well points 60 by opening the valved connection of the wellpoints to pipe 52, or indirectly through bed 50 and pots 48 into outletpipe 52, and pumped to a holding tank. In the case of the dewateringprocess, the dewatered solids are then physically removed by mechanicalmeans out of the tank for transport elsewhere. For example, where largescale tanks are used, access may be provided to the interior of the tankby a manned compact track loader or skid-steer loader to physically digout and transport the dewatered solids. Access to the tank may be, forexample, by a removable sealed door.

EXAMPLE 2 Fine Mineral Feed

Effective dewatering of a fine mineral feed by using a prepared coarsemineral bed material in the agitated tank was achieved. Two tests wererun, one in which no coarse bed material was added and one in which 4 kgof coarse bed material was added to the tank. The fine mineral feed usedwas a residue of cyanide-leached shaking table tailings. It was a fineparticle gradation, finely graded whereby all particles were less than 1mm in diameter and there was a P80 of 370 microns (80% of materialpassed a screen of 370 microns). The following parameters were observedand measured, where “charge weight” is the total weight of solids(including the seed filter bed material), “wt bed material (%)” is thepercent by weight of the bed material to the total charge weight, “flocaddition (g) is the number of grams of flocculant added, “pulp density(%) is the weight percent of solid to the total weight of the slurry,“settling time” is the time in minutes after cessation of agitation andprior to draining, and “drain time” is the time in minutes fromcommencement of draining to when there is no standing water above thebed and no further liquid flowing through the well points.

Fine Mineral Feed Test Number 1 2 Charge weight (kg) 6 10 Wt BedMaterial (%) 0 40 Floc Addition (g) 25 25 Pulp Density (%) 40 40Settling Time (min) 15 15 Drain Time (min) >300 45

From this test it is shown that, without coarse bed material, 6 kg ofthe fine mineral feed took >300 minutes to naturally dewater though thedrain in the bottom of the tank. When 4 kg of coarse bed material wasadded to the tank the total time for dewatering (such that no standingwater was present above the material) was reduced to 45 minutes.

EXAMPLE 3 Dewatering of Organic Matter (Soil)

Effective dewatering of organic matter by using a prepared coarsemineral bed material in the agitated tank was also achieved. Two testswere run, one in which no coarse bed material was added and one in which5 kg of coarse bed material was added to the tank. The organic matterwas standard gardening top soil. A single cycle of agitation, settlingand vacuum was used. The following parameters were measured andobserved.

Organic Matter (Soil) Test Number 1 2 Charge weight (kg) 2.5 5 Wt BedMaterial (%) 0 50 Floc Addition (g) 30 30 Pulp Density (%) 25 25Settling Time (min) 45 45 Drain Time (min) 300 15

From this test it is shown that that, without coarse bed material, 2.5kg of the soil feed took 300 minutes to naturally dewater though thedrain in the bottom of the tank. When 5 kg of coarse bed material wasadded to the tank the total time for dewatering (such that no standingwater was present above the material) was cut to 15 minutes.

While a number of exemplary aspects and embodiments have been discussedabove, those of skill in the art will recognize certain modifications,permutations, additions and sub-combinations thereof. It is thereforeintended that the invention be interpreted to include all suchmodifications, permutations, additions and sub-combinations as arewithin their true spirit and scope.

1. (canceled)
 2. Method of separating a liquid from particulate solids,comprising the steps of: i) providing an apparatus for dewateringparticulate solids, comprising: a) a tank for containing saidparticulate solids and liquid as a slurry; b) an input to the interiorof said tank for introduction of said particulate solids and liquid intosaid tank; c) a liquid outlet passage communicating with the interior ofsaid tank; d) an agitator suspended within said tank for forming asuspension of said particles in said liquid; e) said tank having afilter bed zone in a lower part thereof for formation of a filter bed todrain liquid from the tank; and f) a screen pipe extending upwardlythrough said filter bed zone and communicating with said output passageto receive at its upper end a flow of liquid from above said lowersection which is carried downwardly to flow out through said filter bedzone to said outlet passage or directly to said outlet passage; ii)introducing said particulate solids and liquid into said tank; iii)agitating said particulate solids and liquid to form a slurry; iv)ceasing said agitation to allow said slurry to settle, thereby forming afilter bed to drain liquid from the tank; v) using the screen pipe totransfer liquid from above said filter bed to within said filter bed ordirectly to said outlet passage; vi) draining the liquid from saidparticulate solids; and vii) removing the particulate solids.
 3. Methodof claim 2 wherein an amount of coarser particulate solids is added tosaid particulate solids and liquid in said tank, wherein said coarserparticulate solids are suitable for forming a filter bed.
 4. Method ofclaim 2 wherein said screen pipe is a well point or well screen. 5.Method of claim 2 wherein said agitator is a variable speed agitator andsaid agitation is slowed prior to said settling step.
 6. The method ofclaim 2 used for agitation leaching of mineral-bearing or metal-bearingparticles, a significant portion of which may be 1 mm in diameter orgreater, wherein said particulate material comprises gold, silver,copper or uranium and said liquid comprises concentrations of lixiviant.